Paul Bibby, Nick Ralston

THE discovery of an 18-year-old man in rugged bushland in northern Sydney was a chance find by two expert rescue bushwalkers who were walking off-track.

Matthew Allen was reportedly only days from death when he was found by the couple on Saturday in an area of dense bushland known as the Berowra Valley, which lies between Westleigh and West Pennant Hills.

Mr Allen had been missing since November 27.

Found ... Matthew Allen.

He was covered in leeches and mosquito bites, had reportedly lost almost half his body weight, and was suffering from gangrene to his feet and lower legs. He is being treated at Hornsby Hospital, and is stable.

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Rob Murray was hiking with his wife on Australia Day, off the track, when they discovered some of the boy's belongings.

Mr Murray is a member of the Bushwalkers Wilderness Rescue Squad, a group of volunteers that helps police search for missing bushwalkers.

The Murrays had not set off to look for Mr Allen but a squad spokeswoman, Caro Ryan, said members often went to areas of bushland where other walkers did not venture.

One of the Allen family's neighbours said the bushwalkers had been able to identify the man by his belongings, and quickly realised that he lived only a few kilometres away.

''They apparently saw the name and address in his wallet and went and knocked on the door of his house to alert the family,'' the neighbour said. ''One of them called the police, who sent in a rescue team.

''They say another week and he wouldn't have been found alive.''

How Mr Allen survived for so long, apparently without food or water, is unclear, but locals said there was water in a creek.

He had been missing for so long that when locals saw a rescue helicopter fly into the valley on Saturday, they thought it was collecting water to fight a nearby bushfire.